In most cases, blocking diodes do not make sense. The fully illuminated current, all-the-time 1 volt diode drop sums to more net loss than partial shaded parallel series array loss. If you have panels in parallel at drastically different direction facing, like on east and west side of house, then blocking diodes will reduce loss but you should really have separate MPPT controllers on each array facing different directions.
A panel in 250-400 watt range has a typical shunt leakage resistance in the 300 ohm range but can be as low as 100 ohms or higher than 1000 ohms.
That is about 0.1 to 0.2 amps of illumination maximum current bled away on paralleled array from a shaded string.
If you have a 4 series panels with two series panels stacks in parallel and one of them is shaded, it becomes a 0.1 to 0.2 amp bleed on other fully illuminated series stack. You lose 0.15A x 144v = 21.6 watts when one of paralleled series array is shaded (in addition to shaded string loss). You lose 2 parallel strings x 1vdc x 8.3 amps = 16.6 watts all the fully illuminated time from blocking diodes on each series string. Unless you have a lot of shading time you usually end up with more cumulative loss from blocking diodes than taking the shaded time parasitic bleed current loss.
You also need some hefty heat sinking on blocking diodes to keep them from burning out from heat generation. The little diodes in the MC4 inline packages have poor reliability due to lack of sufficient heat sinking.
